What is referred to as a completion operation relates to operations of preparation or outfitting necessary for bringing a geologic formation into production from the wellbore. These completion operations use particular fluids called completion fluids.
What is referred to as a workover operation relates to an operation performed in a producing or a potentially producing well. Workover fluids can be used in the producing well in circulation in a comparable way to drilling fluids or in the form of a spacer fluid.
All these drilling, completion or workover fluids have in common the physico-chemical adaptation of their formulation to the nature of the geologic formations with which they are in contact and to the main functions for which they are intended. In fact, it is well-known that the fluids used in a well generally have most of the following functions:
maintenance of the stability of the well walls, PA1 controlled filtration in the permeable formations, PA1 good capacity for cleaning the bottom of the well in circulation, PA1 easy density control, PA1 temperature stability and stability in time, PA1 no or little change in properties through contamination by clays or electrolytes, PA1 easy manufacture and treatment, etc. PA1 for a polymer having a molecular mass less than about 10.sup.5 daltons, R.sub.4 contains at least two carbon atoms, PA1 for a polymer having a molecular mass ranging between about 10.sup.5 and 2.5 10.sup.6 daltons, R.sub.4 contains at least four carbon atoms. PA1 acrylamide according to the following formula: ##STR2## where R.sub.5 is H or CH.sub.3, and Z.sub.1 is CONH.sub.2, and possibly acrylic acid, acrylate or sulfonate comonomers according to the following formula: ##STR3## where R'.sub.5 is H or CH.sub.3, and Z.sub.2 is COOH or COO--, M.sup.+ or CONHR.sub.1 SO.sub.3 --, M.sup.+ ; R.sub.1 is H or a C.sub.1 -C.sub.30 alkyl, aryl or alkyl-aryl radical.
In order to achieve these complex combined functions which may be incompatible, the relative quantities of additives must be selected and adjusted. To that effect, according to the nature of the problems encountered in the geologic formations, to their bottomhole conditions, such as pressure and temperature, and according to the main functions essential to the fluid, the composition of the well fluid is generally the result of a compromise between several functions and the cost of the essential additives.